Agree with what you're saying, I think the ANCAP system is flawed in that the star rating of each car if only applicable to when it was tested.
However the Mustang failed the crash test with poor structural performance, it was awarded 2 stars (16%) in 2017. Then in 2018 driver aids were fitted and even with all the active driving aids it only achieved 3 stars (61%). Pretty poor result even noting the ANCAP systems flaws.
https://www.carsales.com.au/editorial/details/ford-mustang-gets-three-star-ancap-rating-113027/
I did own one until recently, it was a nice enough car, got plenty of stares, but the build quality was poor. Paint and panel alignment was the worst I'd seen in a long time. The Coyote V8 engine is also suffering from "rattles" and "ticks" in many cars and Ford are reporting it as normal. Owners are reporting the noise can get so loud that other people notice it at traffic lights. Workshops are stating it is likely piston slap. Bit of a worry.
https://www.cjponyparts.com/resources/mustang-engine-tick